Merriam-Webster lists all three of those as accepted pronunciations [0]; like many things in English, there is significant regional variation.

[0] http://www.merriam-webster.com/dictionary/quay

Is the word "Quay" even used in US English?

https://en.wikipedia.org/wiki/Quay_(disambiguation) gives quays all over the UK, Canada, New Zealand, Australia and Singapore, but none in the US -- with the exception of a couple of tiny landlocked towns in Oklahoma and New Mexico called "Quay".

Are we allowed to call a mispronunciation a mispronunciation when it's only pronounced that way by people who have probably never encountered that word before?

Care to map the phonics path from quay to "key"? "kway" seems like the most reasonable pronunciation unless the letters are just meant to be strokes in a pictogram.

That's because what tantalor left out is that anglophones started misspelling that word in the 17th century, when it was actually spelled "key" from old French "kay".

So it's not so much a word that you go "ah I'm so clever I can pronounce it right", but more one that you go "some people along the way started misspelling this too often and the people who knew better didn't do a good job in correcting them".

And why should I know how to pronounce a word that current anglophones don't know how to spell? It's a tongue-in-cheek question, people.

http://youpronounce.it/search.jsp?q=niche is also a bit like that.